R-Core Transformers
R-Core Transformers are unique in that they’re manufactured using a rectangular core with a round cross-section known as an R-Core. These special cores feature a continuous strip of grain-oriented silicon steel with no air gap. The winding is done on special round bobbins on the two parallel legs of the core.
These clever transformers can be designed so that coil windings mutually cancel each other’s magnetic flux through balanced, symmetrical winding on the two legs. This winding method minimizes the leakage flux.
Additionally, R-Core transformers have absolutely no gap in the magnetic path. This feature also allows leakage flux to be extremely low. EI-type, UI-type, and cut core-type transformers have 2 to 6 magnetic gaps, and a great deal of leakage flux escapes from these magnetic gaps. Leakage flux in an R-Core Transformer is dramatically lower: about 1/10th that of conventional EI or UI transformers.
The R-Core, with its low leakage and low noise characteristics, is ideally suited for medical equipment, audio and video amplifiers, and other noise-sensitive equipment. Circuit designers can place the transformer closer to other critical electronic components.
Another unique feature of the R-Core is the use of two-part bobbins. The primary and secondary windings are done on separate concentric bobbins, ensuring complete isolation. This allows the R-Core transformer to meet all safety standards requirements.
Comparison of the Three Types of Transformers
| Characteristic | R-Core | Toroidal | El Type |
|---|---|---|---|
| Leakage Flux | Balanced winding cancels the leakage flux, so the total amount is extremely small. As a result, it can be used without a shield. | While there are no gaps in the magnetic path, the winding is not balanced, so there’s still some leakage flux. Shielding is often required to reduce the effect on other components. | There are gaps in the magnetic path, and the winding is not balanced. As a result, the leakage flux is very high, which can affect sensitive electronic components if proper shielding is not provided. |
| Winding | Winding is done on a special machine, resulting in evenly-spaced winding. Balanced winding is an inherent constituent of the design. | The winding is not evenly spaced. The density of turns on the inner edge is higher, and the wires on the outer edge are roughly spaced out. Thus, the winding is not balanced. | While the layers of turns are well laid out, the winding is only on the center limb of the EI core. As a result, it is not balanced. |
| Exciting Current | The exciting current is minimal because all the magnetic paths are in alignment with the steel's rolling direction, and there is no gap in the core. | The exciting current is less than that in EI transformers but is generally higher than that of R-Core transformers due to the windings not being balanced. | Due to the presence of magnetic gaps, a more exciting current is required, which results in the inability to use the effect of the rolling direction of grain-oriented steel, variation in assembly operation, etc. |
| Efficiency | Very low losses result in better efficiency. An efficiency of greater than 90% is generally observed. | Efficiency is better than an EI Transformer but generally less than an R-Core transformer. | More losses result in poor efficiency. |
| Mounting | Due to their inherently thin design and light weight, the R-Core transformers can be mounted flat, vertically, or on their sides. | Because of the circular shape, mounting options are limited. | Due to its cubic shape and heavier weight, the EI transformer takes up more space in the assembly. |
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Custom Magnetics, Inc. is a premier manufacturer of R-Core Transformers, the latest choice for many mid- to high-end applications. We’re a trusted partner in a wide range of industries and applications for virtually all types of custom-designed and manufactured transformers and magnetics components, including the innovative R-Core Transformer. For more information on what Custom Magnetics can do for you, or if you’re interested in requesting a quote on your next project, don’t hesitate to contact us today to get started.